1. Field of the Invention
The present invention relates to a control circuit and a switching power supply unit.
2. Description of the Related Art
Ripple injection non-linear control in which a comparator is used is known as a method of controlling a switching power supply unit (see U.S. Pat. Nos. 6,147,478 and 7,202,642).
In the circuit illustrated in FIG. 9 that is used in ripple injection hysteresis control described in U.S. Pat. No. 6,147,478, a comparison voltage Vfb input to the non-inverted input terminal of a comparator 106 becomes a first voltage, which is at a high level, when an output Sp from the comparator 106 is high and becomes a second voltage, which is at a low level, when the output Sp from the comparator 106 is low. A difference between the first voltage and the second voltage is a hysteresis width. An alternating-current component obtained by integrating the output voltage from the comparator 106 or a voltage in phase with the output voltage is superimposed on a direct-current voltage obtained by dividing an output voltage V2 by using a resistor, after which the resulting voltage is input to the inverted input terminal of the comparator 106. If the input voltage is lower than a comparison voltage for the second voltage, the output Sp from the comparator 106 goes high and a driving period starts, in which case the comparison voltage becomes the first voltage. The alternating-current voltage, which has been input to the inverted input terminal of the comparator 106, is raised during the driving period, and is superimposed on the direct-current voltage obtained by dividing the output voltage by using a resistor. If the resulting voltage is higher than a comparison voltage for the first voltage, the output Sp from the comparator 106 goes low and the driving period is terminated, entering a stop period. At that time, the comparison voltage becomes the second voltage. In the stop period, the alternating-current voltage, which has been input to the inverted input terminal of the comparator 106, is lowered.
In ripple injection non-linear control in which a comparator is used, a control method is also known in which a ripple is input to the non-inverted input terminal of a comparator, besides a control method in which a ripple is input to the inverted input terminal of a comparator. In the circuit illustrated in FIG. 10 that is used in non-linear control described in U.S. Pat. No. 7,202,642, for example, a triangle-wave alternating-current voltage is superimposed on a comparison voltage and the resulting voltage is input to the non-inverted input terminal of the comparator 106. The voltage input to the non-inverted input terminal is compared with the direct-current voltage of an output voltage, which has been input to the inverted input terminal. If the voltage at the non-inverted input terminal becomes higher than a voltage fed back from the output voltage, the output from the comparator 106 goes high and goes low soon. There is non-linear control in which a fixed on-time is provided; in non-linear control of this type, the driving period is stopped when a certain period elapses after a high-level trigger signal for the output Sp is received from the comparator 106. Conversely, there is non-linear control in which a fixed off-time is provided.
As a non-linear control method in which a comparator is used, Japanese Unexamined Patent Application Publication No. 2007-181389 discloses a technology about burst control in which switching is performed a plurality of times during a period in which a comparator is at the high level. The technology in Japanese Unexamined Patent Application Publication No. 2007-181389 includes a hysteresis comparator that compares an output voltage with a comparison voltage and a circuit that turns off a gate voltage for a certain period when a current in a switching element reaches a certain value. The gate voltage is tuned on during a period in which the hysteresis comparator is at the high level, after which, when the current reaches the certain value, the gate voltage is tuned off and a period during which the hysteresis comparator is at the high level is continued. Therefore, the gate voltage is turned on again, after which, when the current reaches the certain value again, the gate voltage is turned off. Since these operations are repeated, switching is performed a plurality of times during a period in which the hysteresis comparator is at the high level. During a period in which the hysteresis comparator is at the low level, the gate voltage is not turned off, so switching is stopped.
During the period in which the hysteresis comparator is at the high level, switching is repeated, so a current larger than a load current is supplied from a switching power supply unit to an output capacitor, raising the output voltage. During the period in which the hysteresis comparator is at the low level, switching is stopped, so the load current is supplied from the output capacitor without a current being supplied from the switching power supply unit, lowering the output voltage.